The lungs may be characterized as a mass exchanger in which oxygen is delivered through the alveoli to blood and carbon dioxide is removed from the blood for exhalation. The efficiency of the lungs, in terms of the exchange of gaseous materials at the blood/gas interface, is dependent in-part on the ventilation of each lung. The term “ventilation” refers to the movement of or the exchange of oxygen-rich air from outside the body into the lung where the air is mixed with relatively oxygen deficient air through the course of breathing. The ventilation function of a patient's lungs can be determined and monitored by measuring the resistance and compliance of the airways of the lung.
The resistance and compliance within different regions of the lungs affect the distribution of pulmonary ventilation. The term “resistance” refers to the flow resistance due to an obstruction or a restriction within a respiratory passageway to the passage or flow of a gas to and from the lungs. The measured unit of resistance is H2O/(liters/sec). The term “compliance” refers to the flexibility or elasticity of the lungs as they expand and contract during a respiratory cycle. The measured unit of compliance is liters/(cm-H2O). If one multiplies resistance and compliance, the unit of time (e.g., seconds) that remains is referred to as a “time constant.” Therefore, the ventilation function of a patient's lungs can be represented by a single value, that is, the time constant.
Spirometry is one technique used to diagnose and monitor respiratory disease. In spirometry, the patient inhales as deeply as possible, and then exhales until all air is completely expelled from the lungs. As one can imagine, this requires a great deal of concentration and effort by the patient, and thus, spirometry readings largely depend on how well the patient is feeling and breathing on a given day. Spirometry measures only the flow volume of air that is inhaled and/or exhaled by the patient. Spirometry does not measure resistance or compliance, and does not rely on the calculation of time constants to determine lung function or lung ventilation. Also, spirometry is relatively insensitive for measurements of small airways and thus has limited use for diagnosis of respiratory diseases in these areas of the lung such as asthma and emphysema. Consequently, spirometry is a relatively insensitive technique for monitoring and diagnosing the most prevalent of respiratory diseases.
U.S. Pat. No. 6,135,105 by Lampotang et al. describes a method of classifying each lung by measuring variations in pressure or flow rates using an invasive endotrachael tube equipped with a pressure sensor. Time constants are computed as the product of measured resistance and compliance. However, it is known that this procedure does not accurately account for convective transport in the small airways of the lung, and thus, can result in significant errors in measurement. Consequently, the accuracy of the resistance and compliance values, and thus, the calculated time constants measured using this procedure are questionable. Also, one cannot ignore the need for the endotrachael tube and the resulting discomfort of the patient due to this invasive procedure.
As a result, there is a need for an apparatus and method for correctly diagnosing and accurately monitoring respiratory disease in a patient without relying on the patient's ability to breath on a particular day or without having to insert an invasive device (e.g., endotrachael tube) into the patient.